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Beyond Mendel. Beyond Mendel. Despite the importance of Mendel’s work, it would be a mistake to characterize the principles he discovered as “laws” because there are important exceptions to most of them. Not all genes show simple patterns of dominant and recessive alleles.

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Beyond mendel1
Beyond Mendel

  • Despite the importance of Mendel’s work, it would be a mistake to characterize the principles he discovered as “laws” because there are important exceptions to most of them.

    • Not all genes show simple patterns of dominant and recessive alleles.

    • In most organisms, genetics is more complicated, because the majority of genes have more than two alleles.

    • In addition, many important traits are controlled by more than one gene.

Incomplete dominance
Incomplete Dominance

  • A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype.

Incomplete dominance1
Incomplete Dominance

  • For example, in cross-pollination experiments between red and white snapdragon plants the resulting offspring are pink.


  • A number of human traits are the result of 2 types of alleles that are equally dominant.

  • Such traits are said to be codominant for that trait.


  • When an individual is heterozygous for such traits, the resulting phenotype or expression of these two traits is a blending, because both traits are expressed equally.


  • The alleles for curly hair and straight hair are examples of alleles for a trait that are codominant.

  • Individuals with curly hair are homozygous for curly hair alleles.

  • Individuals with straight hair are homozygous for straight hair alleles.

  • Individuals who are heterozygous, with one of each allele have wavy hair, which is a blend of the expressions of the curly and straight hair alleles.

Multiple alleles
Multiple Alleles

  • Diploid organisms naturally have a maximum of 2 alleles for each gene expressing a particular characteristic, one deriving from each parent.

  • In some cases, however, more than two types of allele can code for a particular characteristic, as is the case of genetic coding for blood type in humans or coat color in rabbits.

  • Multiple alleles result from different mutations of the same gene.

Multiple alleles1
Multiple Alleles

  • Coat color in rabbits is determined by four alleles.

  • Human ABO blood types are determined by alleles A, B, and O.

  • A and B are codominants which are both dominant over O.

Multiple alleles2
Multiple Alleles

  • The only possible genotype for a type O person is ii.

  • Type A people have either IAIA or IAi genotypes.

  • Type B people have either IBIB or IBi genotypes.

  • Type AB have only the IAIB (heterozygous) genotype.

Multiple alleles3
Multiple Alleles

  • The A and B alleles of gene I produce slightly different glycoproteins (antigens) that are on the surface of each cell.

  • Homozygous A individuals have only the A antigen, homozygous B individuals have only the B antigen, homozygous O individuals produce neither antigen, while a fourth phenotype (AB) produces both A and B antigens.

Rh blood type system
Rh Blood type system

  • When we are told our blood type, it is usually expressed as a letter followed by either a positive (+) or negative (-). 

  • This + or - refers to the Rh system of blood testing and is usually quoted in combination with the ABO system described earlier in this lecture:

  • e.g. AB+ or O-

Rh blood type system1
Rh Blood type system

  • The Rh system is actually much more complicated than the ABO system because there are more than 30 combinations possible when inherited, however for general usage, the Rh proteins are grouped into two families:

  • either positive (+) or negative (-).

Rh blood type system2
Rh Blood type system

  • As with the ABO system, there is a dominant allele which happens to be the positive family. 

  • This means that the genetic pairs that can exist in humans are as follows:

    Genotype Blood type

    ++ Rh +

    +- Rh +

    -- Rh -

Polygenic traits
Polygenic Traits

  • Traits controlled by two or more genes are called polygenic traits.

Polygenic traits1
Polygenic traits

  • Usually polygenic traits are distinguished by:

    • Two or more gene pairs contribute to the phenotype.

    • Traits are usually quantified by measurement rather than counting.

    • Phenotypic expression of polygenic traits varies over a wide range.

Polygenic traits2
Polygenic Traits

  • Human polygenic traits include:

    • Height

    • Weight

    • Eye Color

    • Intelligence

    • Skin Color

    • Many forms of behavior